Typically, a solid state laser cavity contains a host material that is doped with a small amount of an activator ion. This doped material is generally called the laser gain medium (LGM) of a device. The LGM can be pumped by a light source such as a flash lamp or more commonly, a diode laser of suitable frequency. The light from the pump is absorbed by the LGM creating a population inversion that causes stimulated emission of coherent light. The LGM is typically placed between two or more mirrors that internally reflect the pump light and create standing waves of the coherent light. The mirrors can be in the form of external self-standing objects with the LGM between them or dielectric coatings deposited directly on the faces of the LGM crystal. This arrangement forms the resonator cavity of a device. The output light can be in the form of continuous or pulsed emission of coherent light.
Coherent light is generally understood to be photons that all have the same wavelength with the same phase and propagating along the same vector. Thus, all of the light emitted from the laser cavity desirably propagates in the same direction. This coherent stimulated emission is in contrast to the random, non-directional propagation that results from spontaneous emission of photons that can also occur in the LGM during the lasing process.
The wavelength of spontaneously emitted photons is very nearly identical to that of those emitted as part of the lasing process. Thus, as the spontaneously emitted photons pass through the lasing crystal, they can serve to induce further random emission from the upper lasing state, which can deplete the population inversion necessary for lasing. In addition, the randomly propagating spontaneously emitted photons can reflect from the edges of the crystal and back into the LGM, further inducing random emission from the excited state and further depopulating the upper lasing levels. This effect is termed amplified spontaneous emission (ASE) and can seriously reduce the power and efficiency of the laser gain medium. In addition, efficient reflectance of spontaneously emitted photons from the edges of the LGM can induce randomly oriented standing waves, creating modes that severely deplete the upper lasing states. This effect is called parasitic oscillation and is an offshoot of ASE. Both phenomena can have a negative effect on laser performance. In fact, severe depletion of power and efficiency of the laser device can occur if ASE is not suppressed.
ASE can be particularly acute in high power systems such as thin disk lasers, devices with very demanding performance standards like microlasers (e.g., end-pumped microlasers) that employ relatively low levels of pumping power to maintain compactness, and systems that include LGM with long path lengths (e.g., side bonded lasers, slab lasers, zig zag lasers, etc.). The effect can be particularly severe for three-level or quasi-three level lasing where it is more difficult to maintain a population inversion in the upper lasing level. Quasi-three level laser ions have considerable thermal population of the lower lasing state since it is not energetically well separated from the actual ground state. Therefore, to achieve population inversion a high pump power is necessary, and ASE contribution to the depopulation of the upper lasing level can severely decrease lasing performance. In addition, the extra pumping power necessary to achieve population inversion adds increased thermal load to the system.
Attempts to eliminate or minimize ASE and parasitic oscillations include development of systems that allow for absorbance of the photons resulting from the spontaneous emission. Since these photons have essentially the same wavelength as the desired lasing wavelength, any absorbing species must be on the edge of the LGM and not in the path of the pump beam or the wave path of the lasing action. This region containing the absorbing ion is often called edge cladding and can be formed of the same material as the LGM host material or a different material. Typically it is doped with a metal ion that can absorb the ASE photons with subsequent thermal relaxation. Another variation on this concept is for the edge cladding of the LGM to be formed to have a significantly larger concentration of the actual activator ion than is doped in the lasing region of the LGM. The large concentration of the activator ion insures that it will absorb the ASE photons but never achieve population inversion so will generally relax by a benign process like thermal emission. These approaches have led to the development of LGM that include different zones in the LGM.
Several approaches have been directed to development of an LGM that includes edge cladding whereby the edge cladding is doped with a different metal ion than the lasing ion or a different concentration of the lasing ion than in the central lasing region of the LGM. Most of these involve some variation of diffusion bonding of a separate edge cladding slab to the central LGM solid. Meissner, et al. (J. App. Phys. 1987, 62, 2647) describe a process for diffusion bonding of a vitreous lead aluminosilicate edge cladding doped with Cu2+ ions to an Nd3+ doped garnet host, Gd3Sc2Ga3O12 (GSGG). The Cu2+ ions in the edge cladding absorb photons of 1064 nm wavelength, which is the lasing wavelength of the Nd3+ ions, and these are responsible for ASE and parasitic oscillations in the Nd:GSGG LGM.
U.S. Pat. No. 7,382,818 to Sumida, et al. describes diffusion bonding of slabs directly to an LGM. Multiple slabs are diffusion bonded in succession after corners and edges are cut off so they can be polished flat before another slab is diffusion bonded to the LGM. U.S. Pat. Nos. 5,441,803 and 5,846,638, both to Meissner, describe a general method of bonding separate slabs together through thermal diffusion for formation of composite optical and electro-optical devices. U.S. Pat. No. 4,849,036 to Powell, et al. describes laser discs that include edge cladding, with the edge cladding bonded to the LGM using an epoxy resin. U.S. Pat. No. 7,609,741 and U.S. Published Patent Application No. 2010/0009475 both to Vetrovec describe the sintering of preforms of ceramic shapes to form a product having distinct zones.
Several criteria are of importance for the successful combination of separate central and cladding regions in an LGM. For instance, in addition to being able to absorb the stray spontaneously emitted photons to prevent their amplification, the edge cladding must have a similar refractive index as the main body of the LGM. If the refractive index difference between the cladding and the central body of the LGM is too large, the ASE photons may not penetrate into the edge cladding and may instead reflect off the interface back into the central portion of the LGM and induce ASE.
The edge cladding must have additional similar physical characteristics as the remainder of the LGM, including such factors as thermal expansion, thermal conductivity and optical damage threshold. Otherwise, the cladding material may crack or separate from the remainder of the LGM during lasing operation. Also the cladding must be robust toward the heat from the pump beam and absorption.
What are needed in the art are methods for forming LGM that can exhibit little or no ASE. For instance, what are needed are LGM that successfully incorporate multiple crystal regimes. A low temperature, facile process that can provide a monolithic heterogeneous LGM including an ASE suppression regime, for instance at an edge cladding, that has similar physical characteristics as is found in the interior of the LGM (e.g., in the path of the pump beam or the wave path of the lasing action) for use in a laser cavity would be of great benefit.